RCBC-160 Explain how increment works with negative values

Change-Id: Ic3132265438f283b7682b1ef18d91092252ffc8a
Reviewed-on: http://review.couchbase.org/32543
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>